S Wave Anelastic Attenuation of Shallow Sediments in Mainland China

Abstract

Frequency-dependent anelastic attenuation is a fact that must be considered when inverting seismic data. The shallow attenuation characteristics of large sedimentary basins with frequent earthquakes in mainland China are still unclear, which hinders the study of strong ground motion. Borehole seismometers distributed in these areas provide information on shallow structures. In this study, the direct and surface-reflected phases in local earthquake borehole seismograms were extracted to calculate the average S wave anelastic attenuation of shallow sediments in mainland China. Relying on the seismograms recorded by 103 borehole stations, the attenuation values and lithologies of the shallow sediments above the borehole seismometers were estimated by applying a multi-time window analysis method. We suggested that the (Formula presented.) values generally decrease from 0.04–0.50 at 1 Hz to 0.01–0.05 at 10 Hz. We obtained the empirical relationships among attenuation, velocity, and depth and found that the dominant mechanisms for attenuation above and below a depth of 400 m could be different due to sediment compositions and scales. Certain borehole depths and corresponding Vs values were corrected by analyzing the time difference variation and the (Formula presented.) distinction.